What is the Sloan Digital Sky Survey?

Simply put, the Sloan Digital Sky Survey is the most ambitious astronomical
survey ever undertaken. The survey maps one-quarter of the entire sky in
detail, determining the positions and absolute brightnesses of hundreds of millions of celestial objects. It also measures distances to more than a
million galaxies and quasars.

The SDSS addresses fascinating, fundamental questions about the universe.
With the survey, astronomers will be able to see the large-scale patterns of
galaxies: sheets and voids through the whole universe. Scientists have many
ideas about how the universe evolved, and different patterns of large-scale structure
point to different theories. The Sloan Digital Sky Survey will tell us
which theories are right - or whether we will have to come up with entirely new ideas.

Mapping the Universe

Making maps is an activity central to the step-by-step advance of
human knowledge. The last decade has seen an explosion in the scale and
diversity of the mapmaking enterprise, with fields as disparate as genetics,
oceanography, neuroscience, and surface physics applying the power
of computers to recording and understanding enormous and complex new
territories. The ability to record and digest immense quantities of data in a
timely way is changing the face of science. The Sloan Digital Sky Survey brings this
modern practice of comprehensive mapping to cosmography, the
science of mapping and understanding the universe.

The SDSS will make the largest map in human history. It will give us a
three-dimensional picture of the universe through a volume one hundred
times larger than that explored to date. The SDSS also records the
distances to 100,000 quasars, the most distant objects known, giving us
an unprecedented hint at the distribution of matter to the edge of the visible universe.
The SDSS is the first large-area survey to use electronic light detectors, so the
images it produces will be substantially more sensitive and accurate than
earlier surveys, which relied on photographic plates. The results of the SDSS
are electronically available to the scientific community and the general public,
both as images and as precise catalogs of all objects discovered. By the end of the survey, the total quantity of information produced will rival
the information content in all the books of the Library of Congress.

By systematically and sensitively observing a large fraction of the sky,
the SDSS will have a significant impact on astronomical studies as diverse
as the large-scale structure of the universe, the origin and evolution of
galaxies, the relation between dark and luminous matter, the structure of our
own Milky Way, and the properties and distribution of the dust from which stars
like our sun were created. The SDSS will be a new reference point, a field
guide to the universe that will be used by scientists for
decades to come.

The Science of the SDSS

The universe today is filled with sheets
of galaxies that curve through mostly empty space. Like soap bubbles in a sink,
they form into dense filaments with voids between. Our best model for how
the universe began, the Big Bang, gives us a picture of a universe filled with
a hot, uniform soup of fundamental particles. Somehow, between the time
the universe began and today, gravity has pulled together the matter into
regions of high density, leaving behind voids. What triggered this change
from uniformity to structure? Understanding the origin of the structure
we see in the universe today is a crucial part of reconstructing our cosmic history.

Understanding the arrangement of matter in the universe is made more difficult because
the luminous stars and galaxies that we see are only a small part of the total.
More than 90% of the matter in the universe does not give off light. The nature,
amount and distribution of this "dark matter" are among the most important
questions in astrophysics. How has the gravity from dark matter influenced
visible structures? Or, put another way, we can use careful mapping of the positions and
motions of galaxies to reconstruct the distribution of mass, and from that, we
can find clues about dark matter.

A Map of the Universe

One of the difficulties in studying the entire universe is getting enough
information to make a picture. Astronomers designed the Sloan Digital Sky Survey
to address this problem in a direct and ambitious way: the SDSS gathers a body of
data large enough and accurate enough to address a broad range of astronomical questions.

The SDSS has obtained high-resolution pictures of one quarter of the
entire sky in five different colors. From these pictures, advanced image
processing software will measure the shape, brightness, and color of hundreds of
millions of astronomical objects including stars, galaxies, quasars (compact but
very bright objects thought to be powered by material falling into giant black
holes), and an array of other celestial exotica. Selected galaxies, quasars, and
stars are observed using an instrument called a spectrograph to determine accurate
distances to a million galaxies and 100,000 quasars, and to provide a wealth of
information about the individual objects. These data give the astronomical
community one of the things it needs most: a comprehensive catalog of the
constituents of a representative part of the universe. SDSS's map will reveal how
big the largest structures in our universe are, and what they look like.
It will help us understand the mechanisms that converted a uniform
"primordial soup" into a frothy network of galaxies.

An Intergalactic Census

The U.S. Census Bureau collects statistical information about how many people
live in the U.S., where they live, their races, their family incomes, and other
characteristics. The Census becomes a primary source of information for people
trying to understand the nation. The Sloan Digital Sky Survey will conduct a sort
of celestial census, gathering information about how many galaxies and quasars
the universe contains, how they are distributed, their individual
properties, and how bright they are. Astronomers will use this information to
study questions such as why flat spiral galaxies are found in less dense regions
of the universe than football-shaped elliptical galaxies, or how quasars have
changed during the history of the universe.

The SDSS will also collect information about the Milky Way galaxy and
even about our own solar system. The wide net cast by the SDSS telescope
will sweep up as many stars as galaxies, and as many asteroids in our solar
system as quasars in the universe. Knowledge of these objects will help us learn
how stars are distributed in our galaxy, and where asteroids fit into the
history of our solar system.

Needles in a Haystack, Lighthouses in the Fog

Rare objects, almost by definition, are scientifically interesting. By
sifting through the several hundred million objects recorded by the SDSS,
scientists will be able to construct entire catalogs of the most distant
quasars, the rarest stars, and the most unusual galaxies. The most unusual
objects in the catalog will be about a hundred times rarer than the rarest
objects now known.

For example, stars with a chemical composition very low in
metals like iron are the oldest in the Milky Way. They can therefore tell us
about the formation of our galaxy. However, such stars are also extremely rare,
and only a wide-field deep sky survey can find enough of them to form a coherent
picture.

Because they are so far away, quasars can serve as probes for intergalactic
matter throughout the visible universe. In particular, astronomers can identify
and study galaxies by the way they block certain wavelengths of light emitted by
quasars behind them. Using the light from quasars, the SDSS will detect tens of
thousands of galaxies in the initial stages of formation. These galaxies are
typically too faint and too diffuse for their own light to be detected by even
the largest of telescopes. Quasar probes will also allow scientists to study the
evolution of the chemistry of the universe throughout its history.

The Telescope as a Time Machine

Peering into the universe with a telescope allows us to look not only out
into space, but also back in time. Imagine intelligent beings in a planetary
system around a star 20 light years away. Suppose these beings pick up a stray
television transmission from Earth. They would see events 20 years after they
occurred on Earth: for instance, a newscast covering Ronald Reagan's re-election
(1984) would be seen 20 years later (2004).
While today we have seen three new presidents, the beings would still
see Reagan.

Light travels extremely fast, but the universe is a very big place. In fact,
astronomers routinely look at quasars so far away that it takes
billions of years for the light they produce to reach us. When we look at
galaxies or quasars that are billions of light-years away, we are seeing them as
they were billions of years ago.

By looking at galaxies and quasars at different distances, astronomers can
see how their properties change with time. The SDSS will measure the
distribution of nearby galaxies, allowing astronomers to compare them with
more distant galaxies now being seen by the new instruments like the Hubble Space
Telescope and the Keck Telescope. Because quasars are very bright, the SDSS
will allow astronomers to study their evolution through more than 90
percent of the history of the universe.

Measuring Distance and Time: Redshift

The universe is expanding like a
loaf of raisin bread rising in an oven. Pick any raisin, and
imagine that it's our own Milky Way galaxy. If you place yourself on that raisin, then
no matter how you look at the loaf, as the bread rises, all the other
raisins move away from you. The farther away another raisin is from you, the faster
it moves away. In the same way, all the other galaxies are moving away from
ours as the universe expands. And because the universe is uniformly expanding,
the farther a galaxy is from Earth, the faster it is receding from us.

The light coming to us from these distant objects is shifted toward the red
end of the electromagnetic spectrum, in much the same way the sound of a train
whistle changes as a train leaves or approaches a station. The faster a distant
object is moving, the more it is redshifted. Astronomers measure the amount of
redshift in the spectrum of a galaxy to figure out how far away it is from
us.

By measuring the redshifts of a million galaxies, the Sloan Digital Sky
Survey provides a three-dimensional picture of our local neighborhood of the
universe.